This invention relates to polyorganosiloxane polymers (silicones) which have acrylic functionality thereon. As used herein, acrylic functionality refers to groups of the structure CH.sub.2 .dbd.C(R)--C(.dbd.O)--O--, where R is H or alkyl.
Acrylic functional silicones are desirable because they have faster and more free radical cure characteristics than do the commercially available vinyl silicones. As described in U.S. Pat. Nos. 2,956,044 and 4,035,355, methyacrylate functional silcones can be formulated with other unsaturated monomers to give monomer compositions or cured polymers with unique and desirable properties. U.S. Pat. No. 4,035,355 describes anaerobically curing compositions of methyacrylate functional siloxane polymers. U.S. Pat. No. 3,577,264 describes radiation curable film-forming paint binders utilizing acrylate or methacrylate functional siloxanes. Other patents describing methods of preparation or uses for methacrylate functional silicones include U.S. Pat. Nos. 2,793,223; 2,898,361; 2,922,806; 2,922,807, and 4,348,454 and U.K. Pat. Nos. 1,384,898 and 1,323,869.
In U.S. Pat. No. 3,878,264 there are described acrylate and methacrylate functional silicones prepared by polymerization of hydrolyzable acrylic functional silanes. Methacrylated silanes may be prepared by hydrosilation of acrylic esters of certain olefinic alcohols. Alternatively, acrylate and methacrylate functional silanes may be prepared by reacting an alkoxy or hydroxy chloroalkyl silane with a tertiary amine salt of acrylic or methacrylic acid.
An especially desirable method of preparing acrylic functional silicones is the one step hydrosilation of acrylic esters of unsaturated alcohols with SiH functional silicones. However hydrosilation of allyl methacrylate with SiH functional silicones consistently yields a product in which about 30% of the methacrylate groups grafted onto the polymer are hydrolyzable. The hydrolyzable groups result from propane elimination which produces methacrylate groups bound directly to silicone through a Si--O--C bond. The presence of these hydrolyzable methacrylate groups produces a number of problems when the polymer is exposed to moisture, including loss of methacrylate functionality and increase in viscosity of the uncured polymer due to siloxy crosslinking.
Allyl acrylate cannot be successfully hydrosilated with SiH functional silicones because addition occurs at both the allyl and the acrylate double bonds. Likewise, addition at both sites occurs in the hydrosilation of methallyl methacrylate.
In U.S. Pat. Nos. 3,555,051 and 3,746,734 there is described the preparation and hydrosilation of propargyl methacrylate with SiH functional silanes to produce alkylenyl methacrylate functional silanes, useful as adhesion promoters and sizing agents. These references also suggest that the method can be used with propargyl acrylate and further suggest preparation of homopolymers having the unit formula: ##STR2## as adhesion promoters, by polymerization of the silane. Such a polymer includes acrylic functionality on each backbone silicon atom.
In copending application Ser. No. 509,568, there are described novel silicone polymers which can be obtained by the one-step hydrosilation of beta (allyloxy)ethyl methacrylate without the problem of propene elimination or addition to both unsaturated sites. Acrylic silicones produced by this method, however, are often difficult to stabilize after synthesis. Accordingly, it is desirable that alternative methods be found for the addition of acrylic functionality to SiH functional silicones.